1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for preventing vehicular wheels from skidding during braking operation and thereby optimise vehicular efficiency and maintain satisfactory stability. More specifically, the invention relates to a novel technology for deriving a vehicular body speed data for performing anti-skid brake control depending upon wheel slippage.
2. Description of Background Art
It is known that the vehicular brake performance becomes optimum when wheel slippage is maintained 10 to 20%. In order to maintain the wheel slippage in the aforementioned optimum range, braking pressure is adjusted over one or more skid control cycles for cyclically increasing and decreasing braking pressure for decelerating a rotation speed of a vehicular wheel. In general, a skid control cycle is controlled based on wheel slippage and wheel acceleration. The wheel acceleration can be derived on the basis of wheel speed. On the other hand, wheel slippage is generally derived on the basis of the wheel speed and a vehicle speed. Therefore, in order to precisely control the skid control cycle, it is essential to determine the vehicle speed.
In the anti-skid brake control technology, it is usual to derive the vehicle speed data on the basis of wheel speed. In the alternative, vehicle speed data is derived based on a longitudinal acceleration exerted on the vehicular body. In the later case, vehicle speed data can be derived by subtracting integrated value of the longitudinal acceleration from an initial wheel speed upon initiation of braking operation. Such an anti-skid brake control technology has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 57-11149.
In such an conventional anti-skid brake control system, the vehicle body speed data is derived based on a wheel speed latched during variation of wheel speed toward locking and an integrated longitudinal acceleration value is integrated from the latching timing of said wheel speed. When integration of the longitudinal acceleration extends for a relatively long period, errors contained in a longitudinal acceleration indicative signal due to secular variation of gain of a longitudinal acceleration sensor, DC offset of the longitudinal acceleration sensor and so forth, or due to vehicular driving conditions such as a hill climbing state, influences controlling the skid control cycle. The influence of the error contained in the longitudinal acceleration indicative signal will become greater as the integration period is expanded.
The influence of the error in the longitudinal acceleration indicative signal will thus degrade accuracy or precision of brake control.